1. Field of the Invention
This invention relates to the field of balanced modulators and, in particular, to balanced modulators for use in switched capacitor auto zeroed networks.
2. Background Art
When implementing operational amplifiers in integrated circuits, ideal characteristics are difficult to achieve. Ideally, when a zero voltage is applied between the inputs of an operational amplifier, the output voltage is zero. In actual amplifiers, however, process variations which result in an imperfect matching of components within the amplifier lead to a positive or negative output voltage when the inputs are forced to zero. The magnitude of the output voltage is directly proportional to the open loop gain of the amplifier and is called output offset voltage.
In order to eliminate the effect of offset voltage, a scheme called "auto zeroing" is utilized. In auto zeroing, a two phase switching scheme is used. In one phase, an input voltage Vin is inputted to one input of an operational amplifier through a capacitor, with the other input being tied to a reference voltage (typically ground). During the second phase, both inputs are tied to ground to compensate for the effect of offset voltage and to automatically "zero" the amplifier. Thus, input voltage signals are only available (valid) during one phase of operation.
In signal processing for communications applications, it is often desired to modulate an input signal from one frequency band to another frequency band by multiplying the input signal by a second signal such as a simple waveform (generally sinusoidal or square). Typically, operational amplifiers are utilized to integrate the input signal with the modulating signal to result in a modulated output signal. Analog switched capacitor technology has been found to be very effective in implementing such modulators on integrated circuit chips. To provide a modulator which is immune to the effects of offset voltage variations, auto-zero networks are often employed. However, because the input signal is coupled to the amplifier at only one phase, the output signal from the amplifier is typically valid at only one phase.
One particular type of modulator is balanced modulator. One application of a balanced modulator is to suppress a carrier of a transmission signal. If a balanced modulator is combined with a low pass filter, a single side band (SSB) signal may be generated.
Prior art balanced modulators utilizing switched capacitor networks require an input signal at both phases of a clock. Thus, these prior art balanced modulators are not suitable for use with auto-zero networks where, as has been pointed out, the input signal is usually available at only one phase.
Therefore, it is an object of the present invention to provide a balanced modulator for use in an auto-zero network.
It is further object of the present invention to provide a balanced modulator which will operate when an input signal is only available at one clock phase.
It is yet another object of the present invention to provide a balanced modulator which will operate when an input signal is only available at one clock phase.
It is yet another object of the present invention to provide a switching scheme which is suitable for use in auto-zero networks in which an input signal is available at only one phase.
It is yet another object of the present invention to provide a switching scheme which is suitable for use in auto-zero networks in which an input signal is available at only phase.